Spinal fusion, also known as spondylosyndesis, is a surgical procedure in which two or more vertebrae are fused together to stop the motion between them. Spinal fusion can be used to treat various pathological and/or traumatic conditions, including, for example: injury to the vertebrae; protrusion and/or degeneration of the intervertebral disc between vertebrae (“slipped” disc or herniated disc); abnormal curvatures of the spine (such as scoliosis or kyphosis); and a weak or unstable spine caused by infections or tumors. Spinal fusion can eliminate motion between vertebral segments, which can be a significant source of pain in some patients. The surgery can also stops the progress of spinal deformity, such as scoliosis.
Some approaches to spinal fusion include implanting a bone fusion device, or interbody cage, in the intervertebral space between adjacent vertebrae. Bone fusion devices can be used to distract adjacent vertebrae away from each other, or expand a collapsed disc space between two vertebrae. Restoring height to collapsed disc spaces can relieve painful pressure on nerves. Such devices can stabilize the vertebrae by preventing them from moving relative to each other while fusion occurs. Bone fusion devices can provide a space for inserting bone growth promotion material such as bone grafts and other bone growth promoting agents between adjacent vertebrae. Over time, the vertebrae and bone graft can grow together through and/or around the device so as to fuse the vertebrae.
Conventional bone fusion devices can have various configurations and may be implanted and/or operated in a variety of ways. For example, conventional bone fusion cages can be cylindrical, rectangular, elliptical, tapered, or other shapes. Such conventional devices may be hollow and can include openings through which bone growth promotion material can contact adjacent bone. Insertion of a bone fusion implant may be accomplished through an open surgical procedure through a relatively large incision. Alternatively, a bone fusion implant may be inserted using a minimally invasive surgical procedure, for example, through percutaneous insertion. Certain conventional bone fusion devices include external threads so that the device can be threaded into adjacent vertebrae having been drilled and tapped for that purpose.
Some conventional bone fusion devices comprise cylindrical cages having a width substantially equivalent to the height of the cage. Although larger heights may be clinically indicated, wider implants are generally not desirable since increased width requires removal of more bone for access to the intervertebral space, which can lead to decreased stability, and more retraction of nerve roots, which can lead to temporary or permanent nerve damage.
Other conventional bone fusion devices include vertebral support components (for example, plates) that are movable from a collapsed state to an expanded state. Such support plates may allow the width of the device to be varied so as to accommodate vertebrae of various sizes. These devices have disadvantages. For example, the support plates may require expansion prior to insertion, or the plates may be operatively connected by externally disposed linkage mechanisms, either of which can cause the device to have dimensions requiring an undesirably large incision for (minimally invasive) delivery to an intervertebral space. Other devices may be expandable after being inserted, but can be difficult to operate in a restricted space such as a collapsed intervertebral space.
Conventional bone fusion devices can involve other difficulties or be associated with other less desirable results. For example, some conventional fusion devices are designed to be impacted into the intervertebral space, which can lead to difficulty in placing the device in a desired position, and can unnecessarily traumatize the vertebral bodies or surrounding nerve and/or vascular tissue. Some of the interbody fusion devices rely on gravity alone to stabilize the device between vertebrae, which can lead to undesirable motion between the vertebrae and difficulty in achieving a complete fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of the devices are not structurally strong enough to support the heavy loads and bending forces at certain levels of the spine, in particular, the lumbar spine. The designs of some of bone fusion cages allow “stress-shielding” of the bone within the cage. Since bone growth is enhanced by stressing or loading the bone material, such “stress-shielding” can greatly increase the time for complete bone growth, or disturb the quality and density of the ultimately formed fusion mass.
Thus, what is desired is a bone fusion device that can be inserted in a minimally invasive manner, that is easily deployed, that provides strong and stable support between adjacent vertebrae, and that promotes optimal bone growth and spinal fusion.